regurgitator 0.8.0 - modernizing to Rack 2.x

I don't think anybody cares or uses this project,
but we now require Rack 2+ and Ruby 2.1+.

6 changes since 0.7.0:

      pkg.mk: avoid network for "gem install"
      use monotonic clock for time comparisons
      gemspec: bump dependencies
      local_file: avoid deprecated Rack::Utils.byte_ranges
      reduce autoload footprint
      fix loading regressions